The molecular pathogenesis of Y. pestis in relevant animal models has been relatively neglected because of the scarcity of secure BSL-3 facilities and trained personnel. The threat of bioterrorism and the emergence of multiply-antibiotic resistant strains of Y. pestis increases the urgency for a more detailed understanding of the host-pathogen relationship at the molecular level that may lead to the design of improved medical countermeasures and diagnostics. RML is one of the few sites in the world where plague pathogenesis can be comprehensively studied at the molecular level. The objective of this project is to establish mouse and rat models of bubonic plague that incorporate flea-to-rodent transmission to investigate the role of specific Y. pestis virulence factors and to characterize the host response to naturally acquired infection. We have established a mouse model to study dissemination and pathogenesis of Y. pestis after flea-borne transmission. In this model, one or more infected fleas are allowed to feed on a restrained mouse. In initial trials, the average number of Y. pestis transmitted by an infectious fleabite was ascertained, an important epidemiologic parameter for which few data are available. The number of bacteria contained in skin biopsies of bite sites was determined. Individual blocked fleas exhibited considerable variability in the number of Y. pestis they introduced into the dermis: only 12 of 48 (25%) skin biopsies contained any bacteria, and the number of Y. pestis in the positive samples ranged from 5 to 7x10^3 (avg. = 1000). From a practical standpoint, it was important to determine the extent of flea to flea variation so that it can be taken into account in planning future transmission studies. In this initial study, we also established that bacteria injected by needle or fleabite into the dorsal posterior dermis disseminated to and could be recovered from dissected inguinal lymph nodes. We have also established a rat model of bubonic plague and characterized the kinetics, microbiology, and histopathology of bubonic plague in rats following intradermal injection of Y. pestis.